Light emitting element and light emitting device

ABSTRACT

A high efficient white emission light emitting element having peak intensity in each wavelength region of red, green, and blue is provided. Specifically, a white emission light emitting element having an emission spectrum that is independent of current density is provided. A first light emitting layer  312  exhibiting blue emission and a second light emitting layer  313  containing a phosphorescent material that generates simultaneously phosphorescent emission and excimer emission are combined. In order to derive excimer emission from the phosphorescent material, it is effective to disperse a phosphorescent material  323  having a high planarity structure such as platinum complex at a high concentration of at least 10 wt % to a host material  322 . Further, the first light emitting layer  312  is provided to be in contact with the second light emitting layer  313  at the side of an anode. Ionization potential of the second light emitting layer  313  is preferably larger by 0.4 eV than that of the first light emitting layer  312.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/793,861, filed Mar. 8, 2004, now allowed, which claims the benefit ofa foreign priority application filed in Japan as Serial No. 2003-072275on Mar. 17, 2003, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting element that comprisesan anode, a cathode, and a layer containing an organic compound(hereinafter, electroluminescent layer) that generates light by applyingelectric field through the electrodes; and a light emitting device thatcomprises the light emitting element. Specifically, the presentinvention relates to a light emitting element that exhibits whiteemission and a full color light emitting device comprising the lightemitting element.

2. Related Art

A light emitting element emits light when electric field is appliedthereto. The emission mechanism is a carrier injection type. That is, byapplying voltage through a pair of electrodes that interposes anelectroluminescent layer therebetween, electrons injected from a cathodeand holes injected from an anode are recombined within theelectroluminescent layer to form molecules in excited states(hereinafter, excited molecule), and the excited molecules return to theground state while radiating energy to emit photon.

There are two excited states possible from organic compounds, thesinglet state and the triplet states. Light emission from the singletstate is referred to as fluorescence and the same from the triplet stateis referred to as phosphorescence.

In such light emitting element, an electroluminescent layer is generallyformed to have a thickness of below 1 μm. Further, since a lightemitting element is a self-luminous element in which anelectroluminescent layer emits photon, a back light used for theconventional liquid crystal display device is unnecessary. Therefore, alight emitting element has a great advantage of being manufactured tohave a ultra thin film thickness and light weight.

In the case of an electroluminescent film with a thickness ofapproximately 100 nm, the time between the injection of carriers andtheir recombination is about several ten nanoseconds considering thecarrier mobility. Hence, the time required for the process of injectingcarriers and emitting light of the electroluminescent layer is on theorder of microsecond. Thus, an extremely high response speed is one ofthe advantages thereof.

Further, since a light emitting element is carrier injection type, itcan be driven by a direct current voltage, thereby noise is hardlygenerated. With respect to a drive voltage, an electroluminescent layeris formed into a uniform ultra thin film having a thickness ofapproximately 100 nm, and a material for an electrode is selected toreduce a carrier injection barrier. Further, a hetero structure(two-layers structure) is introduced. Accordingly, a sufficientluminance of 100 cd/m² can be obtained at an applied voltage of 5.5V(reference 1: C. W. Tang and S. A. VanSlyke, Applied Physics Letters,vol. 51, No. 12, pp. 913-915 (1987)).

A light emitting element has been attracted attention as a nextgeneration's device for a flat panel display in terms of the thinthickness and light weight, the high response speed, the direct lowvoltage operation, or the like. In addition, a light emitting elementcan be used effectively as the device for the display screen of aportable electric appliance in terms of the self luminous type, the wideviewing angle, and the high level of visibility.

Wide variations of emission color is also one of the advantages of alight emitting element. Richness of color is resulted from themultiplicity of an organic compound itself. That is, an organic compoundis flexible enough to be developed to various materials by designingmolecules (such as introducing substituent). Accordingly, a lightemitting element is rich in color.

From these viewpoints, it would not be an overstatement to say that thebiggest application areas of a light emitting element is a full colorflat panel display device. Various means for full colorization have beendeveloped in view of characteristics of a light emitting element. Atpresent, there are three primary methods of forming the structure of afull color light emitting device by using a light emitting element.

First, the method that light emitting elements having three primarycolors, that is, red (R), green (G), and blue (B) are patterned,respectively, by shadow mask technique to serve them as pixels(hereinafter, RGB method). Second, a blue light emitting element is usedas a light emission source, and the blue emission is converted intogreen or red by color changing material (CCM) made from phosphorescentmaterial to obtain three primary colors (hereinafter, CCM method).Third, a white light emitting element is used as a light emissionsource, and a color filter (CF) used for a liquid crystal display deviceor the like is provided to obtain three primary colors (hereinafter, CFmethod).

Of these methods, the CCM method and the CF method do not require suchelaborate patterning required in the RGB method since a light emittingelement used in the CCM method and the CF method exhibits single colorsuch as white (CCM method) or blue (CF method). The CCM materials orcolor filter can be made by the conventional photolithography techniquewithout complicated processes. Further, in addition to these advantageswith respect to processes, the change in luminance with time of eachcolor is uniform since only one kind of device is used.

However, in case of adopting the CCM method, there has been a problem inred color since color conversion efficiency of from blue to red is poorin principle. In addition, there has been a problem that the contrastbecomes deteriorated since a color conversion material itself isfluorescent so that light is generated in pixels due to outside lightsuch as sunlight. CF method has no such problems since a color filter isused as well as the conventional liquid display device.

Accordingly, although the CF method has comparative few disadvantages,the CF method has a problem that a high efficient white light emittingelement is indispensable to the CF method since a great deal of light isabsorbed into a color filter. A mainstream white light emitting elementis the device that combines complementary colors (such as blue andyellow) (hereinafter, two wavelengths white light emitting device)instead of white color having the peak intensity in each wavelength ofR, G, and B (reference 2: Kido et al., “46^(th) Applied Physics RelationUnion Lecture Meeting” p 1282, 28a-ZD-25 (1999)).

However, considering a light emitting device combined with a colorfilter, a white light emitting element having an emission spectrum withthe peak intensity in each wavelength of R, G, and B (hereinafter, threewavelengths white light emitting device) is desirable instead of the twowavelengths white light emitting device, which was reported in thereference 2.

Such three wavelengths white light emitting device has been disclosed insome references (reference 3: J. Kido at al., Science, vol. 267,1332-1334 (1995)). However, such three wavelengths white light emittingdevice is inferior to the two wavelengths white light emitting device interms of luminous efficiency, consequently, significant improvement isrequired. In addition, it is difficult for the three wavelengths whitelight emitting device to obtain stable white emission due to change incolor with time or change in spectrum depending on the current density.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention is toprovide a high efficient white emission device having the emission peakintensity in each wavelength region of red, green, and blue. A furtherobject of the invention is to provide a white emission device having theemission spectrum, which does not depend on the current density.

An even further object of the invention is to provide a light emittingdevice, which operates at lower power and hardly has color shiftcompared with the conventional light emitting device by manufacturing alight emitting device using the light emitting element.

The inventor found out the means for solving the problems after theirearnest consideration by combining a first light emitting layerexhibiting blue emission and a second light emitting layer exhibitingsimultaneously both phosphorescent emission and excimer emission.

A phosphorescent material can convert the triplet excited state to lightemission, that is, generate phosphorescent emission. In a light emittingelement, it is considered that singlet excited state and triplet excitedstate are generated in the ratio of 1:3. Accordingly, it is known thathigh light emission efficiency can be achieved by using a phosphorescentmaterial.

According to the invention, since a second light emitting layer isformed by a phosphorescent material that can generate simultaneouslyboth phosphorescent emission and excimer emission, the second lightemitting layer can exhibit light emission having at least two peaksintensity. Excimer emission is at the longer wavelength side comparedwith the phosphorescent emission. Therefore, by designing the two peaksof intensity to be in the wavelength region of from green to red, a highefficient light emitting device having the peak intensity in eachwavelength region of red, green, blue can be obtained.

The constitution of the invention is a light emitting element thatcomprises, between an anode and a cathode, a first light emitting layerexhibiting blue emission; and a second light emitting layer containing aphosphorescent material, and generating phosphorescent emission from thephosphorescent material and excimer emission from the phosphorescentmaterial, simultaneously.

A first light emitting layer may contain a host material dispersed witha guest material exhibiting blue emission. Therefore, the constitutionof the invention is a light emitting element that comprises, between ananode and a cathode, a first light emitting layer containing a hostmaterial dispersed with a guest material exhibiting blue emission; and asecond light emitting layer containing a phosphorescent material, andgenerating phosphorescent emission from the phosphorescent material andexcimer emission from the phosphorescent material, simultaneously.

The inventor found out that it is effective to disperse a phosphorescentmaterial to a host material at a high concentration of at least 10 wt %in order to derive excimer emission from the phosphorescent material.Therefore, the constitution of the invention is a light emitting elementthat comprises, between an anode and a cathode, a first light emittinglayer exhibiting blue emission; and a second light emitting layercontaining a host material dispersed with a phosphorescent material atconcentration of at least 10 wt %, and generating phosphorescentemission from the phosphorescent material and excimer emission from thephosphorescent material, simultaneously.

In addition, the first light emitting layer may contain a host materialdispersed with a guest material exhibiting blue emission. Therefore, theconstitution of the invention is a light emitting element thatcomprises, between an anode and a cathode, a first light emitting layercontaining a first host material dispersed with a first guest materialexhibiting blue emission; and a second light emitting layer containing asecond host material dispersed with a phosphorescent material atconcentration of at least 10 wt %, and generating phosphorescentemission from the phosphorescent material and excimer emission from thephosphorescent material, simultaneously.

In a light emitting element according to above constitutions, the firstlight emitting layer is provided to be in contact with the second lightemitting layer at a side of the anode, and ionization potential of thesecond light emitting layer is larger by at least 0.4 eV than that ofthe first light emitting layer.

In case that the first light emitting layer contains a host materialdispersed with a guest material exhibiting blue emission, the firstlight emitting layer is preferably provided to be in contact with thesecond light emitting layer at a side of the anode, and ionizationpotential of the second light emitting layer is preferably larger by atleast 0.4 eV than that of the host material in a thin film shape.

In case that the second light emitting layer contains a host materialdispersed with a guest material exhibiting blue emission, the firstlight emitting layer is preferably provided to be in contact with thesecond light emitting layer at a side of the anode, and ionizationpotential of the host material in a thin film shape is preferably largerby at least 0.4 eV than that of the first light emitting layer.

In case that the first light emitting layer contains a host materialdispersed with a guest material exhibiting blue emission and in casethat the second light emitting layer contains a host material dispersedwith a guest material exhibiting blue emission, the first light emittinglayer is preferably provided to be in contact with the second lightemitting layer at a side of the anode, and ionization potential of thesecond host material in a thin film shape is preferably larger by atleast 0.4 eV than that of the first host material in a thin film shape.

In the above constitution of the invention, an emission spectrum withmaximum intensity of the first light emitting device is preferably in awavelength region of at least 400 and at most 500 nm. In addition, thephosphorescent material has an emission spectrum with at least two peaksintensity at a region of at least 500 and at most 700 nm, and either atleast the two peaks is excimer emission, preferably. Further, it iseffective to combine the above emission for obtaining excellent whiteemission.

In the invention, an organic metal complex with platinum as a centralmetal is preferably used as the phosphorescent material.

By manufacturing a light emitting device using the light emittingelement according to the invention, a light emitting device, whichoperates at low power and has hardly color shift, can be provided.Therefore the invention includes a light emitting device using the lightemitting element according to the invention.

Especially, it is effective to apply the light emitting elementaccording to the invention to a full color light emitting device with acolor filter since the light emitting element can achieve high efficientwhite emission having the peak in each red, green and blue wavelength.

As used herein, the term “light emitting device” refers to an imagedisplay device, or the like. Further, a module having a light emittingelement attached with a connector such as FPC (Flexible PrintedCircuit), TAB (Tape Automated Bonding), or TCP (Tape Carrier Package); amodule having TAB or TCP provided with a printed wiring board; and amodule having a light emitting element installed directly with IC(Integrated Circuit) by COG (Chip On Glass) are all included in thelight emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are band diagrams of a light emitting element accordingto the present invention;

FIGS. 2A and 2B are band diagrams of a light emitting element accordingto the invention;

FIG. 3 shows a device configuration of a light emitting elementaccording to the invention;

FIG. 4 shows a device configuration of a light emitting elementaccording to the invention;

FIG. 5 shows specifically a device configuration of a light emittingelement according to the invention;

FIGS. 6A and 6B are schematic views of a light emitting device accordingto the invention;

FIGS. 7A to 7G show examples of electric appliances using a lightemitting device according to the invention;

FIG. 8 shows an emission spectrum according to Example 2 and ComparativeExample 1;

FIG. 9 shows current density dependence of an emission spectrumaccording to Example 2;

FIG. 10 shows luminance vs. current density characteristics according toExample 2 and Comparative Example 1;

FIG. 11 shows luminance vs. voltage characteristics according to Example2 and Comparative Example 1;

FIG. 12 shows current efficiency vs. luminance characteristics accordingto Example 2 and Comparative Example 1; and

FIG. 13 shows current vs. voltage characteristics according to Example 2and Comparative Example 1.

DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the principal of operation and the specificexamples of the device configuration. Although the present inventionwill be fully described by way of examples with reference to theaccompanying drawings, it is to be understood that various changes andmodifications will be apparent to those skilled in the art. Therefore,unless otherwise such changes and modifications depart from the scope ofthe present invention hereinafter described, they should be construed asbeing included therein. Either electrode of a light emitting element maybe transparent since light is extracted from the either electrode.Therefore, not only the conventional configuration that light isextracted from a substrate provided with a transparent electrode, butalso the configuration that light is extracted from the side opposing tothe substrate or the configuration that light is extracted from bothside of the electrode can be applied actually to the light emittingdevice.

The basic concept of the invention is to apply a first light emittinglayer exhibiting blue emission and a second light emitting layercomprising a phosphorescent material, which generates simultaneouslyboth phosphorescent emission and excimer emission.

Excimer emission is always at the longer wavelength side (specifically,at least several ten nm distant from normal emission) compared with thenormal emission (phosphorescent emission in case of a phosphorescentmaterial). Consequently, excimer emission of a phosphorescent materialgenerating phosphorescent emission in green wavelength region is at redwavelength region. Therefore, a high efficient light emitting elementhaving peak intensity in each wavelength region of red, green, and bluecan be achieved by adopting the underlying concept of the invention.

As a first light emitting layer exhibiting the blue emission, a layerformed by a single substance (blue luminous body), or a layer formed bydispersing a guest material serving a blue luminous body to a hostmaterial.

Further, it is necessary that both phosphorescent emission and excimeremission are generated from a phosphorescent material in order tocomplete the invention. Specifically, there is a technique that aphosphorescent material having a high planarity structure such asplatinum complex is used as a guest material, and the phosphorescentmaterial is doped at high concentration (more specifically, at least 10wt %). By doping the phosphorescent material at least 10 wt % inconcentration, mutual action of the phosphorescent material each otheris increased, and excimer emission is derived. Alternatively, thetechnique that a phosphorescent material is not used as a guest materialbut as a thin film light emitting layer or a dotted light emissionregion is acceptable. However, the way of deriving excimer emission froma phosphorescent material is not limited thereto.

From a device configuration perspective, the device configuration isnecessary to be designed to generate light in both the first lightemitting layer and the second light emitting layer. As a means ofdesigning the configuration, the first light emitting layer having holetransportation properties is provided to come in contact with the secondlight emitting layer and to be interposed between the second lightemitting layer and an anode via other layers; and ionization potentialof the second light emitting layer is sufficiently increased comparedwith that of the first light emitting layer.

The band diagram for explaining the principle is shown in FIG. 1A. FIG.1A shows the HOMO level (ionization potential) 110 and the LUMO level111 of a first light emitting layer 101, the HOMO level (ionizationpotential) 112 and the LUMO level 113 of the second light emitting layer102, respectively.

In this instance, in case that energy gap 120 between the ionizationpotential 110 of the first light emitting layer 101 and the ionizationpotential 112 of the second light emitting layer 102 is small, holes arepenetrating from the first light emitting layer 101 into the secondlight emitting layer 102. Then, almost carriers are eventuallyrecombined within the second light emitting layer 102 since the firstlight emitting layer 101 has hole transportation properties.Accordingly, the second light emitting layer 102 exhibits light emissionin green wavelength region and red wavelength region. Consequently, thesecond light emitting layer 102 cannot transfer energy to the firstlight emitting layer 101, which exhibits blue emission at shortwavelength side, and only the second light emitting layer 102 generateslight.

For preventing the phenomenon, the energy gap 120 may be sufficientlyincreased. Accordingly, almost carriers are recombined at the vicinityof the boundary face of the first light emitting layer 101 and thesecond light emitting layer 102. Then, both the first light emittinglayer 101 and the second light emitting layer 102 can exhibit light bythe recombination of the small numbers of carriers within the secondlight emitting layer 102, or the partly energy transfer, the energy isgenerated by the recombination in the first light emitting layer 101,from the first light emitting layer 101 to the second light emittinglayer 102. In addition, the energy gap 120 may be, specifically, atleast 0.4 eV. From some results of experiment, there are many caseswhere both the first light emitting layer and the second light emittinglayer generate light at energy gap 120 of 0.4 eV.

The same is equally true of the case where the configuration that aguest material generating blue emission is dispersed in a host material.That is, the ionization potential of the second light emitting layer 102is preferably larger by at least 0.4 eV than that of the whole firstlight emitting layer 101 (in the state that a guest material generatingblue emission is dispersed in a host material of the first lightemitting layer).

More preferably, the ionization potential of the second light emittinglayer 102 is larger by 0.4 eV than that of the host material of thefirst light emitting layer in a thin film shape. The principle isexplained with reference to band diagram shown in FIG. 1B. FIG. 1B showsthe HOMO level (ionization potential) 114 and the LUMO level 115 of thehost material of the first light emitting layer in a thin film shape,and the HOMO level (ionization potential) 116 and the LUMO level 117 ofa guest material generating blue emission, respectively. Anothercomponents are denoted by like numerals as of FIG. 1A.

In FIG. 1B, holes are transported into the HOMO level (ionizationpotential) 114 of the host material of the first light emitting layer ina thin film shape. Therefore, the energy gap 121 between the ionizationpotential 112 of the second light emitting layer 102 and the ionizationpotential 114 of the host material of the first light emitting layer ina thin film shape is preferably at least 0.4 eV.

The principle with respect to the case of using a phosphorescentmaterial as a guest material will be explained. FIG. 2A is the banddiagram. FIG. 2A shows the HOMO level (ionization potential) 210 and theLUMO level 211 of a first light emitting layer 201, the HOMO level(ionization potential) 212 and the LUMO level 213 of the host materialof a second light emitting layer 202 in a thin film shape, and the HOMOlevel (ionization potential) 214 and the LUMO level 215 of the guestmaterial (phosphorescent material) of a second light emitting layer 202,respectively.

In this instance, as well as the case explained with reference to FIGS.1A and 1B, the ionization potential of the whole second light emittinglayer 202 (in the state that a phosphorescent material is dispersed inthe host material of the second light emitting layer) is preferablylarger by at least 0.4 eV than that of the first light emitting layer201.

More preferably, the ionization potential 212 of the host material ofthe second light emitting layer in a thin film shape is larger by atleast 0.4 eV than the ionization potential 210 of the first lightemitting layer 201.

In such state, almost holes are accumulated at the vicinity of boundaryface of the first light emitting layer 201 and the second light emittinglayer 202 since the energy gap 220 is large. However, a part of theholes are trapped into HOMO level 214 of a phosphorescent material.Therefore both the first light emitting layer 201 and the second lightemitting layer 202 can exhibit light emission.

In case of adopting the configuration that a guest material generatingblue emission is dispersed in a host material to the first lightemitting layer 201, as well as the case described with reference toFIGS. 1A and 1B, the ionization potential of the whole second lightemitting layer 202 (in the state that a phosphorescent material isdispersed in the host material of the second light emitting layer) ispreferably larger by at least 0.4 eV than that of the whole first lightemitting layer 201 (in the state that a guest material generating blueemission is dispersed in the host material of the first light emittinglayer).

More preferably, the ionization potential of the host material of thesecond light emitting layer in a thin film shape is larger by at least0.4 eV than that of the host material of the first light emitting layerin a thin film shape. The principle is explained with reference to aband diagram in FIG. 2B. FIG. 2B shows the HOMO level (ionizationpotential) 216 and the LUMO level 217 of the host material of the firstlight emitting layer in a thin film shape and the HOMO level (ionizationpotential) 218 and the LUMO level 219 of the guest material generatingblue emission. Another components are denoted by like numerals as ofFIG. 2A.

In FIG. 2B, holes are transported into the HOMO level 216 of the hostmaterial of the first light emitting layer in a thin film shape.Therefore, in case that the energy gap between the ionization potential212 of the host material of the second light emitting layer 202 in athin film shape and the ionization potential 216 of the host material ofthe first light emitting layer 201 is at least 0.4 eV, the phenomenondescribed with reference to FIG. 2A is resulted, consequently, both thefirst light emitting layer and the second light emitting layer exhibitlight emission.

Hereinafter, the device configuration of a light emitting elementaccording to the invention will be explained. An electroluminescentlayer of the light emitting element according to the invention comprisesat least the above described first light emitting layer and second lightemitting layer. Further, layers having other properties than lightemission, which are known as the components of the conventional lightemitting element, such as a hole injecting layer, a hole transportinglayer, an electron transporting layer, and an electron transportinglayer can be appropriately included.

Materials that can be used for each the layers will be illustrated inspecific. However, the materials that can be applied to the inventionare not limited thereto.

As a hole injection material for forming a hole transporting layer,porphyrin compounds are useful among other organic compounds such asphthalocyanine (abbreviated H₂-Pc), copper phthalocyanine (abbreviatedCu-Pc), or the like. Further, chemical-doped conductive polymercompounds can be used, such as polyethylene dioxythipophene (abbreviatedPEDOT) doped with polystyrene sulfonate (abbreviated PSS), polyaniline(abbreviated PAni), polyvinyl carbazole (abbreviated PVK), or the like.A thin film of an inorganic semiconductor such as vanadium pentoxide ora ultra thin film of an inorganic insulator such as aluminum oxide canalso be used.

As a hole transportation material for using a hole transporting layer,aromatic amine (that is, the one having a benzene ring-nitrogen bond)compounds are preferably used. For example,N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine(abbreviated TPD) or derivatives thereof such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereafter, referred toas α-NPD) is widely used. Also used are star burst aromatic aminecompounds, including: 4,4′,4″-tris(N,N-diphenyl-amino)-triphenyl amine(hereafter, referred to as TDATA); and4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenyl amine(hereafter, referred to as “MTDATA”).

As electron transportation materials for forming an electrontransporting layer, in specific, metal complexes such astris(8-quinolinolate) aluminum (abbreviated Alq₃),tris(4-methyl-8-quinolinolate) aluminum (abbreviated Almq₃),bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviated BeBq₂),bis(2-methyl-8-quinolinolate)-(4-hydroxy-biphenylyl)-aluminum(abbreviated BAlq), bis [2-(2-hydroxyphenyl)-benzooxazolate] zinc(abbreviated Zn(BOX)₂), and bis [2-(2-hydroxyphenyl)-benzothiazolate]zinc (abbreviated Zn(BTZ)₂). Besides, oxadiazole derivatives such as2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviatedPBD), and 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl] benzene(abbreviated OXD-7); triazole derivatives such as3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated TAZ) and3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated p-EtTAZ); imidazol derivatives such as2,2′,2″-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] (abbreviatedTPBI); and phenanthroline derivatives such as bathophenanthroline(abbreviated BPhen) and bathocuproin (abbreviated BCP) can be used inaddition to metal complexes.

As electron injection material for forming an electron injecting layer,above described electron transportation materials can be used. Besides,a ultra thin film of insulator, for example, alkaline metal halogenatedcompounds such as LiF, CsF, or the like; alkaline earth halogenatedcompounds such as CaF₂ or the like; or alkaline metal oxides such asLi₂O is often used. In addition, alkaline metal complexes such aslithium acetylacetonate (abbreviated Li(acac)), 8-quinolinolato-lithium(abbreviated Liq), or the like can also be used.

As a luminous body in the first light emitting layer, blue fluorescentmaterials having hole transportation properties such as above describedTPD, α-NPD, or the like; or blue fluorescent materials having electrontransportation properties such as Balq, Zn(BOX)₂, or the like. Variousblue fluorescent dyes, for example, perylene, 9,10-diphenyl anthracene,coumarin based fluorescent dyes (coumarin 30 or the like) can be used asa guest material. Further, phosphorescent materials such asbis(4,6-difluorophenyl)pyridinato-N,C²′) (acetylacetonato)iridium(abbreviated Ir(Fppy)₂(acac)) can be used. All of these materials haveemission peak intensity in the wavelength of from 400 to 500 nm, so thatthey are suitable for materials for the luminous body of the first lightemitting layer according to the invention.

As the luminous body of the second light emitting layer, an organicmetal complex, with platinum as the central metal, is effectively used.Specifically, if materials represented by the following structuralformulas 1 to 4 dispersed in a host material in high concentration, bothphosphorescent emission and excimer emission can be derived. However,the invention is not limited thereto, any phosphorescent material can beused, as long as it can generate phosphorescent emission and excimeremission.

In case of using guest materials for the first light emitting layer andthe second light emitting layer, the hole transportation materials orelectron transportation materials typified by the above describedexamples can be used as the host material. In addition, a bipolarmaterial such as 4,4′-N,N′-di carbazolyl-biphenyl (abbreviated CBP) canbe used.

As a material for an anode of the light emitting element according tothe invention, a conductive material having large work function ispreferably used. In case of extracting light through an anode, atransparent conductive material such as indium-tin oxide (ITO),indium-zinc oxide (IZO), or the like may be used for forming the anode.In case of forming an anode to have a light blocking effect, a singlelayered film such as TiN, ZrN, Ti, W, Ni, Pt, Cr, or the like; alamination layered film of a titanium nitride film and a film containingaluminum as its main components; a three layered film of a titaniumnitride film, a film containing aluminum as its main components, and atitanium nitride film can be used for forming the anode. Alternatively,an anode having a light blocking effect can be formed by stacking theabove described conductive material over a reflective electrode such asTi, Al, or the like.

As a material for a cathode, conductive materials having small workfunction is preferably used. Specifically, alkaline metals such as Li,Cs, or the like; alkaline earth metals such as Mg, Ca, Sr, or the like;alloys of theses metals (Mg: Ag, Al: Li, or the like); or rare earthmetals such as Yb, Er, or the like. In addition, in case of using anelectron injecting layer such as LiF, CsF, CaF₂, Li₂O, or the like, theconventional conductive thin film such as aluminum can be used. In caseof extracting light through cathode, alkaline metals such as Li, Cs, orthe like, or a lamination structure comprising a ultra thin filmcontaining alkaline earth metals such as Mg, Ca, Sr, or the like and atransparent conductive film (ITO, IZO, ZnO, or the like). Alternatively,the cathode having a light blocking effect is formed by forming anelectron injecting layer by alkaline metals or alkaline earth metals,and electron transportation materials by co-evaporation and stacking atransparent conductive film (ITO, IZO, ZnO, or the like) thereon.

FIGS. 3 and 4 show examples of the light emitting element according tothe invention. However, the invention is not limited thereto.

FIG. 3 shows the device configuration having an electroluminescent layer302 interposed between an anode 301 and a cathode 303. The devicecomprises an anode 301, a hole injecting layer 311, a first lightemitting layer 312, a second light emitting layer 313, an electrontransporting layer 314, and a hole injecting layer 315, sequentially.Here, the first light emitting layer 312 is composed of a luminous bodyhaving hole transporting properties such as α-NPD. In the second lightemitting layer 313, a phosphorescent material 323 such as platinumcomplexes (represented by above structural formulas 1 to 4) is dispersedin a host material 322 in high concentration (specifically, at least 10wt %). Consequently, both phosphorescent emission and excimer emissionis derived.

By practicing the present invention, a white light emitting elementhaving such simple configuration can be provided having emission peakintensity in each wavelength of red, green, and blue. Further, thedevice shown in FIG. 3 can exhibit stable white emission withoutchanging the shape of emission spectrum even when current density ischanged or the element is continuously operated since only one kind ofdoping material (a phosphorescent material 323) is used to the device.

FIG. 4 shows the configuration having an electroluminescent layer 402interposed between an anode 401 and a cathode 403. The device comprisesan anode 401, a hole injecting layer 411, a hole transporting layer 412,a first light emitting layer 413, a second light emitting layer 414, anelectron transporting layer 415, and a hole injecting layer 416,sequentially. In the first light emitting layer 413, a holetransportation material 421 used for the hole transporting layer 412 isused as host and a blue luminous body 422 such as perylene is used asguest. In the second light emitting layer 414, a phosphorescent material424 such as platinum complexes (represented by above structural formulas1 to 4) is dispersed in a host material 423 in high concentration(specifically, at least 10 wt %). Consequently, both phosphorescentemission and excimer emission is derived.

A method for stacking each layer of the light emitting element accordingto the invention is not limited. Any method for stacking such as vacuumvapor deposition, spin coating, ink jetting, dip coating, or the likecan be used, as long as layers can be stacked by these methods.

EXAMPLES

Hereinafter, examples of the present invention will be explained.

Example 1

In this example, a device configuration of a light emitting element anda method for manufacturing thereof according to the present inventionwill be explained with reference to FIG. 5.

An anode 501 of the light emitting element is formed over a glasssubstrate 500 having an insulating surface. As a material for the anode501, ITO, a transparent conductive film, is used. The anode 501 isformed by sputtering to have a thickness of 110 nm. The anode 501 issquare in shape and 2 mm in height and width.

Then, an electroluminescent layer 502 is formed over the anode 501. Inthis example, the electroluminescent layer 502 has a laminationstructure comprising a hole injecting layer 511; a first light emittinglayer 512, which has hole injection properties; a second light emittinglayer 513; an electron transporting layer 514; and an electron injectinglayer 515. The first light emitting layer 512 is formed by a material,which can achieve blue emission, specifically, a material, which has theemission spectrum with maximum intensity in the wavelength of from 400to 500 nm. In addition, the second light emitting layer 513 is formed bya host material or a guest material that generates phosphorescent lightemission.

First, a substrate provided with the anode 501 is secured with asubstrate holder of a vacuum deposition system in such a way that thesurface provided with the anode 501 is down. Then, Cu-Pc is put into anevaporation source installed in the internal of the vacuum depositionsystem. And then, the hole injection layer 511 is formed to have athickness of 20 nm by vacuum vapor deposition with a resistive heatingmethod.

Then, the first light emitting layer 512 is formed by a material, whichhas excellent hole transportation properties and light-emissionproperties. In this example, α-NPD is deposited in accordance with thesame procedures as those conducted for forming the hole injection layer511 to have a thickness of 30 nm.

And then, the second light emitting layer 513 is formed. In thisexample, the second light emitting layer 513 is formed by CBP as a hostmaterial and Pt(ppy)acac represented by the structural formula 1 as aguest material, which is controlled to be 15 wt % in concentration, tohave a thickness of 20 nm by co-evaporation.

Further, the electron transporting layer 514 is formed over the secondlight emitting layer 513. The electron transporting layer 514 is formedby BCP (bathocuproin) to have a thickness of 20 nm by vapor deposition.CaF₂ is deposited to have a thickness of 2 nm as the electron injectionlayer 515 thereon to complete the electroluminescent layer 502 having alamination structure.

Lastly, a cathode 503 is formed. In this example, the cathode 503 isformed by aluminum (Al) by vapor deposition with a resistive heatingmethod to have a thickness of 100 nm.

Therefore, a light emitting element according to the invention isformed. In addition, in the device configuration described in Example 1,the first light emitting layer 512 and the second light emitting layer513 can exhibit light emission, respectively, so that a device thatexhibits white emission as a whole can be formed.

In this example, an anode was formed over a substrate; however, theinvention is not limited thereto. A cathode can be formed over asubstrate. In this case, that is, in case of exchanging an anode tocathode, lamination sequence of the electroluminescent layer describedin this example is reversed.

In this example, the anode 501 is a transparent electrode in order toextract light generated in the electroluminescent layer 502 from theanode 501; however, the invention is not limited thereto. If the cathode503 is formed by a selected material that is suitable for securingtransmittance, light can be extracted from the cathode.

Example 2

In this example, device characteristics of the light emitting elementdescribed in Example 1 having the configuration: ITO/Cu-Pc (20 nm)/α-NPD(30 nm)/CBP+Pt(ppy)acac: 15 wt % (20 nm)/BCP (30 nm)/CaF (2 nm)/Al (100nm) will be explained. Emission spectrum of the light emitting elementhaving the above described configuration is shown by each spectrum 1 inFIG. 8 and FIG. 9. Each plot 1 in FIGS. 10 to 13 shows for electriccharacteristics.

Spectrum 1 in FIG. 8 shows the emission spectrum of the light emittingelement having the above described configuration at an applied currentof 1 mA (approximately 960 cd/m²). From the result shown by spectrum 1,white emission can be obtained having three components: blue emissionfrom α-NPD composing the first light emitting layer (˜450 nm); greenemission of phosphorescent light emission from Pt(ppy)acac contained ina second light emitting layer (˜490 nm, ˜530 nm); and orange emissionfrom excimer light emission of Pt(ppy)acac contained in the second lightemitting layer. CIE chromaticity coordinate is (x, y)=(0.346, 0.397).The light emission was almost white in appearance.

Ionization potential of the α-NPD used for the first light emittinglayer and the CBP used for the second light emitting layer was measured.The α-NPD had ionization potential of approximately 5.3 eV, and the CBPhad that of approximately 5.9 eV. The difference in the ionizationpotential between the α-NPD and the CBP was approximately 0.6 eV.Therefore, preferable condition of the invention, that is, ionizationpotential of at least 0.4 eV is satisfied. Consequently, it can beconsidered that the fact resulted in good white emission. In addition,the measurement of ionization potential was carried out withphotoelectron spectrometer (AC-2) (RIKEN KEIKI Co., Ltd.).

FIG. 9 shows measurement results of each spectrum at different amount ofcurrent flow in the light emitting element having the above describedconfiguration. FIG. 9 shows measurement results at different amount ofcurrent flow denoted by spectrum a (0.1 mA), spectrum b (1 mA), andspectrum c (5 mA). Clearly from the measurement results, a spectralshape was hardly changed even when the amount of current flow wasincreased (luminance was increased). It can be considered that the lightemitting element according to the invention exhibits stable whiteemission, which is hardly affected by the change of the amount ofcurrent flow.

As electric characteristics of the light emitting element having theabove described configuration, the luminance-current plot 1 in FIG. 10shows that a luminance of approximately 460 cd/m² was obtained at acurrent density of 10 mA/cm².

The luminance-voltage plot 1 in FIG. 11 shows that a luminance ofapproximately 120 cd/m² was obtained at an applied voltage of 9 V.

The current efficiency-luminance plot 1 in FIG. 12 shows that currentefficiency of approximately 4.6 cd/A was obtained at a luminance of 100cd/m².

The current-voltage plot 1 in FIG. 13 shows that a current flow wasapproximately 0.12 mA at applied voltage of 9 V.

Comparative Example 1

Correspondingly, each spectrum 2 and spectrum 3 in FIG. 8 shows emissionspectrum measured from a light emitting element formed by differentconcentration of Pt(ppy)acac contained in a light emitting layer fromthat in Example 1. The spectrum 2 shows a measurement result in the casethat concentration of Pt(ppy)acac is 7.9 wt %. The spectrum 3 shows ameasurement result in the case that concentration of Pt(ppy)acac is 2.5wt %. In each case, the spectrum was obtained at the current flow of 1mA.

As shown by spectrum 3 of FIG. 8, in case that Pt(ppy)acac is containedat concentration of 2.5 wt %, blue emission from α-NPD composing a firstlight emitting layer (approximately 450 nm) and green emission fromPt(ppy)acac contained in a second light emitting layer (approximately490 nm, approximately 530 nm) were only observed, consequently, whiteemission could not be obtained. As shown in spectrum 2, in case thatPt(ppy)acac is contained at concentration of 7.9 wt %, a slight ofexcimer emission was in the spectrum as a shoulder at the vicinity of560 nm; however the peak was insufficient, consequently, excellent whiteemission could not be obtained.

Further, current characteristics were measured from the devices. Eachplot 2 in FIGS. 10 to 13 shows measurement results from the devicecontaining Pt(ppy)acac at concentration of 7.9 wt %. Each plot 3 inFIGS. 10 to 13 shows measurement results from the device containingPt(ppy)acac at concentration of 2.5 wt %.

The luminance-voltage characteristics in FIG. 10 shows that a luminanceof approximately 180 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 7.9 wt % and a luminance ofapproximately 115 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 2.5 wt % at a current density of 10mA/cm², respectively.

The luminance-voltage characteristics in FIG. 11 shows that a luminanceof approximately 93 cd/m² was obtained form the device containingPt(ppy)acac at concentration of 7.9 wt % and a luminance ofapproximately 73 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 2.5 wt % at an applied voltage of 9 V,respectively.

The current efficiency-luminance characteristics in FIG. 12 shows that acurrent efficiency of approximately 1.8 cd/A was obtained from thedevice containing Pt(ppy)acac at concentration of 7.9 wt % and a currentefficiency of approximately 1.1 cd/A was obtained from the devicecontaining Pt(ppy)acac at concentration of 2.5 wt % at the luminance of100 cd/m², respectively.

The current-voltage characteristics in FIG. 13 show that a current flowwas approximately 0.21 mA in the device containing Pt(ppy)acac atconcentration of 7.9 wt % and a current flow was approximately 0.27 mAin the device containing Pt(ppy)acac at concentration of 2.5 wt % at anapplied voltage of 9 V, respectively.

The above measurement results (especially, the result of thecurrent-voltage characteristics) provide the fact that the lightemitting element according to the invention containing Pt(ppy)acac as aguest material in high concentration (15 wt %) has the same level ofelectric characteristics as those of the light emitting elementcontaining Pt(ppy)acac as a guest material in such low concentration(7.9 wt %, 2.5 wt %).

Example 3

In this example, a method for manufacturing a light emitting device (topemission structure) having a light emitting element according to thepresent invention, which exhibits white emission, over a substratehaving an insulating surface will be explained with reference to FIG. 6.As used herein, the term “top emission structure” refers to a structurein which light is extracted from the opposite side of the substratehaving an insulating surface.

FIG. 6A is a top view of a light emitting device. FIG. 6B is across-sectional view of FIG. 6A taken along the line A-A′. Referencenumeral 601 indicated by a dotted line denotes a source signal linedriver circuit; 602, a pixel portion; 603, a gate signal line drivercircuit; 604, a transparent sealing substrate; 605, a first sealingagent; and 607, a second sealing agent. The inside surrounded by thefirst sealing agent 605 is filled with the transparent second sealingagent 607. In addition, the first sealing agent 605 contains a gap agentfor spacing between substrates.

Reference 608 denotes a wiring for transmitting signals inputted to thesource signal line driver circuit 601 and the gate signal line drivercircuit 603. The wiring receives video signals or clock signals from anFPC (flexible printed circuit) 609 serving as an external inputterminal. Although only FPC is illustrated in the drawing, a PWB(printed wirings board) may be attached to the FPC.

Then, a cross-sectional structure will be explained with reference toFIG. 6B. A driver circuit and a pixel portion are formed over asubstrate 610. In FIG. 6B, the source signal line driver circuit 601 andthe pixel portion 602 are illustrated as a driver circuit.

The source signal line driver circuit 601 is provided with a CMOScircuit formed by combining an n-channel TFT 623 and a p-channel TFT624. A TFT for forming a driver circuit may be formed by a known CMOS,PMOS, or NMOS circuit. In this example, a driver integrated type inwhich a driver circuit is formed over a substrate is described, but notexclusively, the driver circuit can be formed outside instead of over asubstrate. In addition, the structure of a using a polysilicon film asan active layer is not especially limited. A top gate TFT or a bottomgate TFT can be adopted.

The pixel portion 602 is composed of a plurality of pixels including aswitching TFT 611, a current control TFT 612, and a first electrode(anode) 613 connected to the drain of the current control TFT 612. Thecurrent control TFT 612 may be either an n-channel TFT or a p-channelTFT. In case that the current control TFT 612 is connected to an anode,the TFT is preferably a p-channel TFT. In FIG. 6B, a cross-sectionalstructure of only one of thousands of pixels is illustrated to show anexample that two TFTs are used for the pixel. However, three or morenumbers of pixels can be appropriately used.

Since the first electrode (anode) 613 is directly in contact with thedrain of a TFT, a bottom layer of the first electrode (anode) 613 ispreferably formed by a material capable of making an ohmic contact withthe drain formed by silicon, and a top layer, which is in contact with alayer containing an organic compound, is preferably formed by a materialhaving a large work function. In case of forming the first electrode(anode) by three layers structure comprising a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, the first electrode (anode) can reduce resistance as a wiring,make a favorable ohmic contact, and function as an anode. Further, thefirst electrode (anode) 613 can be formed by a single layer such as atitanium nitride film, a chromium film, a tungsten film, a zinc film, ora platinum film; or a lamination layer composed of three or more layers.

Insulator (also referred to as a bank) 614 is formed at the edge of thefirst electrode (anode) 613. The insulator 614 may be formed by anorganic resin film or an insulating film containing silicon. In thisexample, an insulator is formed by a positive type photosensitiveacrylic film as the insulator 614 in the shape as illustrated in FIG.6B.

In order to make favorable coverage, an upper edge portion and a loweredge portion of the insulator 614 are formed to have a curved facehaving a radius of curvature. For example, positive type photosensitiveacrylic is used as a material for the insulator 614, only upper edgeportion of the insulator 614 is preferably have a radius of curvature(from 0.2 to 3 μm). As the insulator 614, either a negative typephotosensitive resin that becomes insoluble to etchant by light or apositive type photosensitive resin that becomes dissoluble to etchant bylight can be used.

Further, the insulator 614 may be covered by a protective film formed byan aluminum nitride film, an aluminum nitride oxide film, a thin filmcontaining carbon as its main component, or a silicon nitride film.

An electroluminescent layer 615 is selectively formed over the firstelectrode (anode) 613 by vapor deposition. Moreover, a second electrode(cathode) 616 is formed over the electroluminescent layer 615. As thecathode, a material having a mall work function (Al, Ag, Li, Ca; oralloys of these elements such as Mg: Ag, Mg: In, or Al: Li; or CaN) canbe used.

In order to pass light, the second electrode (cathode) 616 is formed bya lamination layer of a thin metal film having a small work function anda transparent conductive film (ITO, IZO, ZnO, or the like). A lightemitting element 618 is thus formed comprising the first electrode(anode) 613, the electroluminescent layer 615, and the second electrode(cathode) 616.

In this example, the electroluminescent layer 615 is formed by alamination structure explained in Example 1. That is, theelectroluminescent layer 615 is formed by stacking sequentially Cu-Pc asa hole injecting layer (20 nm), α-NPD as a first light emitting layerhaving hole transporting properties (30 nm), CBP+Pt(ppy)acac:15 wt % (20nm) as a second light emitting layer, and BCP as an electrontransporting layer (30 nm). In addition, an electron injecting layer(CaF₂) is unnecessary in the device since a thin film metal film havinga small work function is stacked as the second electrode (cathode).

Thus formed light emitting element 618 exhibits white emission. Inaddition, a color filter comprising a coloring layer 631 and a lightshielding layer (BM) 632 is provided to realize full color (forsimplification, an over coat layer is not illustrated).

In order to seal the light emitting element 618, a transparentprotective lamination layer 617 is formed. The transparent protectivelamination layer 617 comprises a first inorganic insulating film, astress relaxation film, and a second inorganic insulating film. As thefirst inorganic insulating film and the second inorganic insulatingfilm, a silicon nitride film, a silicon oxide film, a silicon oxynitridefilm (composition ratio: N<O), a silicon nitride oxide film (compositionratio: N>O), or a thin film containing carbon as its main component (forexample, a DLC film or a CN film) can be used. These inorganicinsulating films have high blocking properties against moisture.However, when the film thickness is increased, film stress is alsoincreased, consequently, film peeling is easily occurred.

By interposing a stress relaxation film between the first inorganicinsulating film and the second inorganic insulating film, moisture canbe absorbed and stress can be relaxed. Even when fine holes (such as pinholes) are existed on the first inorganic insulating film at filmformation for any reason, the stress relaxation film can fill in thefine holes. The second inorganic insulating film formed over the stressrelaxation film gives the transparent protective lamination filmexcellent blocking properties against moisture or oxygen.

A stress relaxation film is preferably formed by a material havingsmaller stress than that of an inorganic insulating film and hygroscopicproperties. In addition, a material that is transparent to light ispreferable. As the stress relaxation film, a film containing an organiccompound such as α-NPD, BCP, MTDATA, or Alq₃ can be used. These filmshave hygroscopic properties and are almost transparent in case of havingthin film thickness. Further, MgO, SrO₂, or SrO can be used as thestress relaxation film since they have hygroscopic properties andtranslucency, and can be formed into a thin film by vapor deposition.

In this example, a silicon nitride film having high blocking propertiesagainst impurities such as moisture or alkaline metals is formed byvapor deposition using a silicon target in the atmosphere containingnitrogen and argon as the first inorganic insulating film or the secondinorganic insulating film. A thin film formed by Alp₃ by vapordeposition as the stress relaxation film. In order to pass light throughthe transparent protective lamination layer, the total film thickness ofthe transparent protective lamination layer is preferably formed to bethin as possible.

In order to seal the light emitting element 618, the sealing substrate604 is pasted with the first sealing agent 605 and the second sealingagent 607 in an inert gas atmosphere. Epoxy resin is preferably used forthe first sealing agent 605 and the second sealing agent 607. It isdesirable that the first sealing agent 605 and the second sealing agent607 inhibit moisture or oxygen as possible.

In this example, as a material for the sealing substrate 604, a plasticsubstrate formed by FRP (Fiberglass-Reinforced Plastics), PVF (polyvinylfluoride), Myler, polyester, acrylic, or the like can be used besides aglass substrate or a quartz substrate. After pasting the sealingsubstrate 604 with the first sealing agent 605 and the second sealingagent 607, a third sealing agent can be provided to seal the side face(exposed face).

By encapsulating the light emitting element 618 in the first sealingagent 605 and the second sealing agent 607, the light emitting element618 can be shielded completely from outside to prevent moisture oroxygen that brings deterioration of the electroluminescent layer 615from penetrating into the light emitting element 618. Therefore a highreliable light emitting device can be obtained.

If the first electrode (anode) 613 is formed by a transparent conductivefilm, a dual light emission device can be manufactured.

The light emitting device according to this example can be practiced byutilizing not only the device configuration of the electroluminescentdevice explained in Example 1 but also the configuration of theelectroluminescent device according to the invention.

Example 4

Various electric appliances completed by using a light emitting devicehaving a light emitting element according to the present invention willbe explained in this example.

Given as examples of such electric appliances manufactured by using thelight emitting device having the light emitting element according to theinvention: a video camera, a digital camera, a goggles-type display(head mount display), a navigation system, a sound reproduction device(a car audio equipment, an audio set and the like), a laptop personalcomputer, a game machine, a portable information terminal (a mobilecomputer, a cellular phone, a portable game machine, an electronic book,or the like), an image reproduction device including a recording medium(more specifically, a device which can reproduce a recording medium suchas a digital versatile disc (DVD) and so forth, and includes a displayfor displaying the reproduced image), or the like. FIGS. 7A to 7G showvarious specific examples of such electric appliances.

FIG. 7A illustrates a display device which includes a frame 7101, asupport table 7102, a display portion 7103, a speaker portion 7104, avideo input terminal 7105, or the like. The light emitting device usingthe light emitting element according to the invention can be used forthe display portion 7103. The display device is including all of thedisplay devices for displaying information, such as a personal computer,a receiver of TV broadcasting, and an advertising display.

FIG. 7B illustrates a laptop computer which includes a main body 7201, acasing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing mouse 7206, or the like. The lightemitting device using the light emitting element according to theinvention can be used to the display portion 7203.

FIG. 7C illustrates a mobile computer which includes a main body 7301, adisplay portion 7302, a switch 7303, an operation key 7304, an infraredport 7305, or the like. The light emitting device using the lightemitting element according to the invention can be used to the displayportion 7302.

FIG. 7D illustrates an image reproduction device including a recordingmedium (more specifically, a DVD reproduction device), which includes amain body 7401, a casing 7402, a display portion A 7403, another displayportion B 7404, a recording medium (DVD or the like) reading portion7405, an operation key 7406, a speaker portion 7407 or the like. Thedisplay portion A 7403 is used mainly for displaying image information,while the display portion B 7404 is used mainly for displaying characterinformation. The light emitting device using the light emitting elementaccording to the invention can be used to the display portion A 7403 andthe display portion B 7404. Note that the image reproduction deviceincluding a recording medium further includes a domestic game machine orthe like.

FIG. 7E illustrates a goggle type display (head mounted display), whichincludes a main body 7501, a display portion 7502, and an arm portion7503. The light emitting device using the light emitting elementaccording to the invention can be used to the display portion 7502.

FIG. 7F illustrates a video camera which includes a main body 7601, adisplay portion 7602, an casing 7603, an external connecting port 7604,a remote control receiving portion 7605, an image receiving portion7606, a battery 7607, a sound input portion 7608, an operation key 7609,an eyepiece portion 7610 or the like. The light emitting device usingthe light emitting element according to the invention can be used to thedisplay portion 7602.

FIG. 7G illustrates a cellular phone which includes a main body 7701, acasing 7702, a display portion 7703, a sound input portion 7704, a soundoutput portion 7705, an operation key 7706, an external connecting port7707, an antenna 7708, or the like. The light emitting device using thelight emitting element according to the invention can be used to thedisplay portion 7703. Note that the display portion 7703 can reducepower consumption of the cellular phone by displaying white-coloredcharacters on a black-colored background.

As set forth above, the light emitting device using the light emittingelement according to the invention can be applied variously to a widerange of electric appliances in all fields. The light emitting devicecan be applied to various fields' electric appliances.

By practicing the present invention, a white light emitting elementhaving high light emission efficiency can be provided. Especially, ahigh efficient white light emitting device, which has the peak intensityin each wavelength region of red, green, and blue, can be provided.Moreover, by manufacturing a light emitting device using the lightemitting element, a light emitting device, which operates at lower powerthan that of the conventional light emitting device, can be provided.

What is claimed is:
 1. A light emitting device comprising: an anode anda cathode; a first light emitting layer between the anode and thecathode, the first light emitting layer comprising a first host materialand a blue-emissive guest material; and a second light emitting layerbetween the first light emitting layer and the cathode, the second lightemitting layer being in contact with the first light emitting layer andcontaining a host material and a phosphorescent material, wherein thephosphorescent material simultaneously generates phosphorescent emissionin green wavelength region and excimer emission in red wavelengthregion, wherein the blue-emissive guest material is dispersed in thefirst host material, and wherein ionization potential of the secondlight emitting layer is larger by 0.4 eV than that of the blue-emissiveguest material.
 2. The light emitting device according to claim 1,wherein the first light emitting layer exhibits blue emission.
 3. Thelight emitting device according to claim 1, wherein the phosphorescentmaterial is an organic metal complex with platinum as a central metal.4. The light emitting device according to claim 3, wherein the organicmetal complex is selected from


5. The light emitting device according to claim 1, wherein the firstlight emitting layer has an emission peak in a wavelength region from400 nm to 500 nm.
 6. The light emitting device according to claim 5,wherein the phosphorescent material has at least two emission peaks at awavelength region from 500 nm to 700 nm, and at least one of theemission peaks corresponds to the excimer emission.
 7. The lightemitting device according to claim 1, wherein the phosphorescentmaterial is dispersed in the host material at a concentration of atleast 15 wt %.
 8. The light emitting device according to claim 1,wherein the light emitting device emits white light.
 9. The lightemitting device according to claim 1, further comprising a holeinjecting layer between the anode and the first light emitting layer.10. The light emitting device according to claim 9, wherein the holeinjecting layer comprises a metal oxide.
 11. An electric appliancecomprising the light emitting device according to claim
 1. 12. Asemiconductor device comprising the light emitting device according toclaim
 1. 13. A light emitting device comprising: a first electrode; afirst light emitting layer over the first electrode, the first lightemitting layer comprising a first host material and a blue-emissiveguest material; a second light emitting layer over and in contact withthe first light emitting layer, the second light emitting layercontaining a host material and a phosphorescent material, and a secondelectrode over the second light emitting layer, wherein thephosphorescent material simultaneously generates phosphorescent emissionin green wavelength region and excimer emission in red wavelengthregion, wherein the blue-emissive guest material is dispersed in thefirst host material, and wherein ionization potential of the secondlight emitting layer is larger by 0.4 eV than that of the blue-emissiveguest material.
 14. The light emitting device according to claim 13,wherein the first light emitting layer exhibits blue emission.
 15. Thelight emitting device according to claim 13, wherein the phosphorescentmaterial is an organic metal complex with platinum as a central metal.16. The light emitting device according to claim 15, wherein the organicmetal complex is selected from


17. The light emitting device according to claim 13, wherein the firstlight emitting layer has an emission peak in a wavelength region from400 nm to 500 nm.
 18. The light emitting device according to claim 17,wherein the phosphorescent material has at least two emission peaks at awavelength region from 500 nm to 700 nm, and at least one of theemission peaks corresponds to the excimer emission.
 19. The lightemitting device according to claim 13, wherein the phosphorescentmaterial is dispersed in the host material at a concentration of atleast 15 wt %.
 20. The light emitting device according to claim 13,wherein the light emitting device emits white light.
 21. The lightemitting device according to claim 13, further comprising a holeinjecting layer over and in contact with the first electrode.
 22. Thelight emitting device according to claim 21, wherein the hole injectinglayer comprises a metal oxide.
 23. An electric appliance comprising thelight emitting device according to claim
 13. 24. A semiconductor devicecomprising the light emitting device according to claim 13.